Human Immunodeficiency Virus-1 (HIV) infection of the central nervous system may cause a neurological syndrome termed HIV-associated neurocognitive disorders (HAND). HAND includes minor neurocognitive disorders and a more severe form of motor and cognitive impairments termed HIV associated dementia (HAD). Although treatment with highly active antiretroviral agents decreases the load of HIV in the brain, the prevalence of HAND is actually increased due to longer life. Therefore, adjunctive and combined therapies must be developed to prevent and perhaps reverse the neurologic deficits observed in these individuals. These subjects exhibit synaptic simplification and neuronal apoptosis. However, the molecular mechanisms leading to synaptic pathology are unknown. Key to developing effective therapies is a better understanding of the molecular and cellular mechanisms by which the virus causes these neuropathological features. HIV and its viral envelope protein gp120 promote axonal retraction and dendritic simplification. These effects are reproduced by the proneurotrophin brain-derived neurotrophic factor (proBDNF) and are mediated by the p75 neurotrophin receptor (p75NTR), a member of the tumor necrosis factor family. Thus, we have generated the hypothesis that proBDNF facilitates HIV neurotoxicity by promoting the activation of p75NTR. We have obtained preliminary data in support to this hypothesis. Indeed, we have shown that brains of HAD subjects exhibit a higher amount of proBDNF than non-HAD subjects. Moreover, in vitro data have determined that HIV and gp120-mediated synaptodendritic simplification is blocked by p75NTR antagonists. Thus, this proposal will test the novel hypothesis that HIV injures neurons by a gp120-mediated mechanism that involves the release of proBDNF, which in turn promotes axonal/dendritic degeneration via a p75NTR-mediated mechanism. Experiments will be carried out to establish the role of p75NTR in HIV/gp120 mediated neurotoxicity, and the cellular mechanisms whereby HIV/gp120 promotes proBDNF accumulation. These include testing the activity of enzymes involved in processing proBDNF to mature BDNF. These experiments will be accompanied by studies examining proteolytic enzymes involved in proBDNF processing in postmortem human brain as well as in animal models of HAD. Studies are also planned to establish the relationship (if any) between proBDNF and microglia activation and inflammation. We expect to provide new significant data on the role of p75NTR in HIV neurotoxicity. These data will help in the design of p75NTR antagonists as an adjunct therapy against synaptic simplification caused by HIV.